Components made of plastic material now account for a share between 15 and 20 percent by weight in automobile construction. These components are frequently exterior and interior parts, wherein the corresponding visual, haptic and functional properties are imparted to the latter by adhesively bonding injection molded components with different surface decors. Plastic parts are used as injection-molded bodies for the interior of automobiles, for example instrument panels, door linings or center consoles. These interior parts may be provided with a decor or painted.
Additionally, competition in the automotive industry toward lightweight construction has resulted in applications for plastic materials, wherein physical foaming, such as the substitution of plastic material with a gas (e.g. nitrogen, CO2), or chemical foaming is also increasingly used. In the case of physical foaming using MuCell® or similar methods, nitrogen or carbon dioxide, for example, is injected in a supercritical state into the plastic melt under pressure during plasticizing and homogenously distributed. After injection into the non-pressurized mold, the gas separates from the melt again and forms a fine-celled foam structure. The elimination of dwell pressure, the reduced viscosity, and minimized or eliminated sink marks and warpage are aspects that intensify the trend.
Foamed polymer objects are known in the technical field and have a variety of applications. Foams are used, for example, for cushioning, insulation, weight reduction, impact absorption, and thermal, chemical and electrical inertness. Thermoplastic polymer foams can be produced using foamed beads or conventional polymer processing techniques, such as extrusion, injection molding, reaction injection molding, and mechanical mixing. Foam extrusion typically includes melting the polymer in an extruder, adding a gas or a compound that is present in a gaseous state at the extrusion temperature and at standard pressure (e.g. blowing agent, foaming agent), or a source of a gas, such as a chemical compound, which generates a gas by way of decomposition, and then extruding the molten thermoplastic polymer through a die to form a foamed structure. Frequently, nucleating agents are also added to the molten polymer, so that the pore size and the homogeneity of the resulting foam are improved.
Methods for producing foamed polymer objects were disclosed, for example, in U.S. Pat. No. 5,726,214, U.S. Pat. No. 4,877,815 and U.S. Pat. No. 3,072,583.
JP 08-012796 discloses the use of a combination of boron nitride and a zeolite as nucleating agents in a composition for foaming fluoropolymer resins.
DE 602 19 054 discloses that zeolites can be used alone, without adding a gas or a chemical bathing agent, to foam a thermoplastic, melt-processable fluoropolymer.
However, the foam molded bodies of the prior art include a variety of drawbacks. For example, a special apparatus is needed for injecting the gas when physical foaming is employed. In addition, nucleating agents and further additives (such as flame-proofing agents) must be added to the composition to produce foams having a small cell size, high homogeneity and wide range of applications, making production more expensive and less practical. A further disadvantage of the method of the prior art is that undesirable outgassing behavior of the foamed molded bodies occurs during and/or after production of foams. This delays the production of lined components and makes them more expensive and less practical. Additionally, when a lining is applied early, the outgassing results in disadvantageous blistering. This requires waiting an undesirably long time for the outgassing to subside before a lining can be applied to the molded body.